This invention relates to a clutch/brake unit in which the clutch is electromagnetically actuated and, more particularly, to a clutch/brake unit which is especially adapted for use with a riding lawnmower, a garden tractor or similar vehicle having an electrical system with a battery and with an alternator for charging the battery. The purpose of the clutch/brake unit is to enable selective engagement and disengagement of an implement drive (e.g., a mower blade drive) and to apply a positive braking torque to the drive when the drive is disengaged.
One type of prior clutch/brake unit used for these purposes includes an electromagnetic clutch having a stationary field shell, an electrical coil disposed within the field shell, a rotor driven by an input member and rotatable relative to the field shell, and an armature connected to an output member and selectively rotatable relative to the rotor. When the coil is energized, magnetic flux threads through the field shell and the rotor and cuts axially across an air gap between the rotor and the armature to attract the armature into engagement with the rotor and thereby couple the output member for rotation in unison with the input member. Upon de-energization of the coil, springs pull the armature away from the rotor to disengage the clutch and allow the input member to rotate relative to the output member. At the same time, the springs pull the armature against a fixed braking member in order to bring the rotating output member to a relatively quick stop.
Because the braking force of such a clutch/brake unit is produced by the springs, the springs must be relatively strong in order to effectively brake the output member. As a result, high magnetic force must be created in order to overcome the springs and effect release of the brake and engagement of the clutch. As the armature moves nearer to engagement with the rotor, the springs become progressively more loaded and exert an even higher force resisting engagement of the armature with the rotor.
Since high magnetic force must be generated in order to overcome the relatively strong springs, the clutch/brake unit must employ a comparatively large electrical coil for creating such force. The use of a large coil requires that the vehicle be equipped with a rather high capacity electrical system for serving the coil as well as the other electrical accessories of the vehicle.